This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. NASA scientists are working to find ways to keep astronauts healthier and safer when they spend a long time in space. Head down bed rest is a good way to mimic a person traveling in space without gravity. Head-down bed rest helps researchers study people on earth in a way that causes some of the changes the body goes through while traveling in space without gravity. In this study scientists will test the use of a mildly vibrating platform to preserve bone quality, muscle strength, and balance during head down bed rest. In order to learn as much as possible during the study, the scientists will measure changes in bone, muscle, heart, circulation, and nervous system, as well as nutrition and immune status. During the study subjects will stay in bed with their head tilted downward for 90 of those days. Along with bed rest, they will have mental and physical testing to measure changes in their body and mind before, during and after bed rest. Every day during the bed rest part of this study subjects will undergo a ten minute vibration procedure. While they are lying in bed the subjects will be held in place with their feet against a mildly vibrating plate by springs attached to a vest that the subject will wear. Scientists hope to get information that will help keep astronauts healthy on future space flights.